The invention relates to tire retreading, and more particularly to an apparatus for curing a retread tire assembly comprising a tire carcass, new tread rubber, and bonding layer.
In tire retreading operations, the worn tread is stripped or buffed from the crown of the tire carcass and new tread rubber is bonded in place. In one type of operation, the new tread rubber is in a cured state when placed on the carcass and a bonding layer of, for example, gum rubber is placed between the crown of the carcass and the cured tread strip. The bonding layer is cured to secure the tread to the carcass. One curing apparatus, an autoclave, requires placing the retread tire assembly in an envelope which is then evacuated. The assembly and envelope are placed in an autoclave for heat and pressure treatment to cure the bonding layer.
In another type of operation, the tread rubber is in uncured state and is applied to the carcass crown and cured in place. The assembly of carcass and uncured tread rubber is placed in a curing press having a bladder that fits within the interior cavity of the tire assembly and heating means to heat the bladder and the area surrounding the tire tread.
An autoclave is a large pressure and temperature controlled vessel having a capacity for several tire assemblies. One model autoclave for curing 25 tire assemblies has an interior volume of 905 cubic feet. Typically, to heat the air in an autoclave, heated steam or oil is forced through a heat exchanger in the autoclave, or electrical resistance elements in the autoclave are used. Each of these has deficiencies.
Steam requires a steam boiler and associated piping and control elements that are expensive to install and maintain. A heated oil system is also expensive to install and maintain, and requires special care to prevent fire hazards. Both steam and heated oil depend on heat transfer to heat the air in the autoclave. Electrical resistance heats the air directly, but is expensive to operate.
There is a need for an apparatus that eliminates the drawbacks in the conventional devices.
The present invention provides an apparatus for curing a retread tire assembly that uses heated, pressurized water as the heating medium for a curing chamber. An apparatus in accordance with the invention is less expensive to install and maintain than either steam or oil systems. The apparatus is less expensive to operate than electrical resistance heating elements.
It had been thought by those skilled in the art that water lacked the heat transfer capacity to sufficiently heat the air in an autoclave chamber to the required temperature for curing the assemblies inside. The inventor has unexpectedly found that heated and pressurized water could operate in a circuit such as a tube and fin heat exchanger to attain and maintain a curing temperature in the autoclave. The invention defines flow rates for the water in the heating system that help achieve this unexpected result.
According to the invention, the apparatus includes an autoclave having an interior space in which retread tire assemblies may be placed for curing. The autoclave has a heat exchanger in the interior space. An electrical or gas powered vessel to heat water is connected in a closed circuit to the heat exchanger. The air in the autoclave is circulated by a fan to ensure uniformity of the air temperature throughout the chamber and to facilitate heat transfer from the heat exchanger.
According to a preferred embodiment of the invention, the vessel has the capacity to heat water to at least 290.degree. F. (143.degree. C.). The vessel preferably has a volumetric capacity of approximately 20 gallons of water.
A pump, preferably a centrifugal pump, is disposed in the closed circuit to pump heated water between the vessel and the heat exchanger under pressure. According to the invention, the pump provides a flow rate of 20 to 50 gallons per minute, and preferably about 25 to 35 gallons per minute.
According to the invention, a control valve maintains a flow of heated water at a constant rate to the autoclave to meet heating demand. A temperature sensor senses the temperature in the autoclave, measuring the air temperature or the temperature at the retread tire assembly, and provides feedback to the control valve for controlling the flow of heated water.
Advantageously, the invention uses the same heat exchanger and control valve as a steam heating system, which minimizes the cost of modifying an existing system to incorporate the invention.
According to an alternative embodiment, the water system is connected to a curing press for curing a retread tire assembly with an uncured tread rubber strip.